Can the meson cloud explain the nucleon strangeness?

Abstract

We use a version of the meson cloud model, including the kaon and the K * contributions, to estimate the electric and magnetic strange form factors of the nucleon. We compare our results with the recent measurements of the strange quark contribution to parity-violating asymmetries in the forward G0 electron-proton scattering experiment. We conclude that it is very important to determine experimentally the electric and magnetic strange form factors, and not only the combination G As new experimental data appear, our picture of the nucleon evolves continually. Our knowledge about the sea quarks in the nucleon has been changing dramatically and, in particular, our ideas about the strange sea quarks have been modified very rapidly. The famous EMC experiment [1] and other polarized DIS experiments [2] could be interpreted as showing that the quarks carry only a small fraction of the total angular momentum of the proton. A further conclusion was that the strange sea quarks in the proton are strongly polarized opposite to the polarization of the proton [3]. The recent results of the HERMES Collaboration [4] indicated that there is a SU(3) symmetry breaking in the nucleon sea. Most of these findings could be well understood with a meson cloud model (MCM) [5][6][7][8][9]. In any version of the meson cloud model, the physical nucleon contains virtual meson-baryon components, that "dress" the bare nucleon. The meson cloud mechanism provides a natural explanation for symmetry breaking among parton distributions [10]. In Ref. [6], it has been shown that the inclusion of the meson cloud significantly lowers the value of the total spin carried by quarks and antiquarks. In Ref.[6] the strange cloud was composed by K and K components in the Fock wave function of the proton and the authors obtained a very small polarization of the strange sea. Later on, in Ref. [11,12], it was shown that the higher mass components K * and K * could have important effects on the strange sea. These components are kinematically suppressed but have large couplings to the nucleon and may lead to a numerically significant contribution to some observables. In particular, the states containing K * affect the quark-antiquark symmetry breaking in the polarized strange sea. When only K mesons were considered it was observed that x[ s(x) − s(x)] > 0. When both contributions of K and K * were included, as it wasComplementary to the high energy regime of Refs. [1,2,4] the nucleon strange sea can be probed in the low energy parity violating experiments carried out at TJNAF, where it is possible to measure the strange electric and magnetic form factors of the nucleon. The first measurements of these quantities (and combinations of them) were performed by the SAMPLE [13] and HAPPEX [14] Collaborations. In this low energy regime the strange component of the nucleon sea is expected to have a nonperturbative origin. One of the possible nonperturbative mechanisms of strangeness production is given precisely by the meson cloud. Indeed, these data were studied in a num...

Comparisons of magnetic charge and axial charge meson cloud distributions in the PCQM

“…The meson cloud of the nucleon, undoubtedly, plays a relevant role in the study of low energy electroweak properties of the nucleon. The meson cloud model, where the nucleon is considered as a system of three valence quarks surrounded by a meson cloud [1][2][3][4][5][6][7][8][9][10], has recently been employed to study the generalized parton distribution [11,12], nucleon electroweak form factors [13][14][15][16][17][18], nucleon strangeness [19,20], etc. In Refs.…”

Isospin violation in the vector form factors of the nucleon

“…In recent years the strange electric and magnetic form factors have been studied experimentally by the SAMPLE [1], A4 [2,3], HAPPEX [4,5], and G0 [6] Collaborations, and theoretically via lattice QCD simulations [7,8,9,10,11,12], chiral perturbation theory [13,14,15,16,17], and hadron models [18,19,20,21,22,23,24,25]. (For a discussion of earlier theoretical studies, see Ref.…”

Additional corrections to the Gross–Llewellyn Smith sum rule

“…It is difficult to imagine a physical mechanism that would cause s À ðxÞ to change sign at such small crossover points x 0 as have been found in these phenomenological analyses [29,30,32,33]. Indeed, model calculations almost invariably yield a zero at x $ 0:1 or higher [38][39][40][41]. The NuTeV group found that with a moderate increase in 2 one could obtain considerably larger values of x 0 and corresponding large decreases in the second moment S À [29].…”